The invention relates to a well-drilling process.
It is known that it would be advantageous to reduce the pressure exerted on a formation during drilling in order to increase the rate of penetration of the drill bit. A requirement may also exist to discharge a small quantity of effluent through a formation during drilling, in order to analyse this effluent without interrupting the drilling. When passing through a loss zone during a drilling operation, a pressure relief in line with this zone would make it possible to continue the drilling in practice without slowing down the drilling operation.
For this purpose, it has already been proposed to divert directly upwards into the annular space included between the well and the drill string, at a certain injection depth, a part of the drilling fluid which travels down in the bore of the drill string and the remainder of which cools the drill bit, cleans the working face and lifts the cuttings to the surface through the annular space, mixing with the portion which is diverted above this injection depth.
However, attempts to employ this process for relieving the drilling fluid pressure locally have not so far met with success, the pressure relief produced failing to reach even a pressure of 10 meters of head of water.
The systematic tests which have led to the present invention have shown that, below a certain threshold flow of drilling liquid injected direclty into the annular space, the pressure-relieving effect was small and showed no or hardly any increase with the injected flow, and was incapable of exceeding 10 meters of head of water, but that, above this threshold, an increase in the injected flow produced a rapid increase in the pressure difference obtained below the injection depth, it being possible for this pressure difference to reach more than 260 meters of head of water.
It is believed that this phenomenon can be explained by the coupling of two effects: a suction effect which is observed at flows below the threshold, and which is very limited, and a thrust effect which appears only above this threshold, but which very rapidly becomes predominant and continues to increase rapidly as the flow is increased.
The earlier failures of attempts to relieve the drilling fluid pressure in the annular space are thus attributed to the ineffectiveness of an increase in flow in the range of flows which have been tried, which did not provide encouragement for the experiments to be continued and did not make it possible to discover that, above a certain threshold flow, things suddenly improved, and that it was possible to obtain very large pressure reliefs.
The thrust effect may be promoted by the fact that the drilling fluid is generally a non-newtonian fluid and that the liquid stream injected upwards into the annular space widens out rapidly and exerts its thrust on a large cross-section in the annular space. On the other hand, this behaviour of the liquid has the disadvantage of making any calculation highly complicated. An approximate mathematical formula based on the experimental results has, nevertheless, been investigated, enabling these results to be accounted for approximately, as long as the pressure relief exceeds 10 meters of head of water, and making it possible to specify the required flow adjustment.